void fitAoZ(){
  const double p[] = {
    1.99968e+00, 2.03745e+00, 2.07674e+00, 2.11859e+00, // 20-Ca40,41,42,43
    2.00245e+00, 2.03728e+00, 2.07398e+00, 2.10963e+00, // 21-Sc42,43,44,45
    2.00288e+00, 2.03617e+00, 2.07195e+00, 2.10895e+00, // 22-Ti44,45,46,47
    2.00246e+00, 2.03548e+00, 2.07020e+00, 2.10574e+00, // 23-V46,47,48,49
    2.00179e+00, 2.03549e+00, 2.06854e+00, 2.10117e+00, // 24-Cr48,49,50,51
    2.03541e+00, 2.06699e+00, 2.09859e+00, // 25-Mn51,52,53
    2.03554e+00, 2.06420e+00, 2.09296e+00, // 28-Ni57,58,59
    2.03565e+00, 2.06219e+00, 2.09006e+00, // 30-Zn61,62,63
    2.06188e+00, 2.08883e+00, // 31-Ga64,65
    2.06072e+00, 2.08620e+00, // 33-As68,69
    2.05955e+00, 2.08405e+00, // 35-Br72,73
    2.05800e+00, 2.08239e+00, 2.14225e+00, // 36-Kr74,75,78
    2.07917e+00 // 37-Rb76
    };
  static const int n = sizeof(p)/sizeof(double);
  double z[n] = {
    40/20., 41/20., 42/20., 43/20., // 20-Ca40,41,42,43
    42/21., 43/21., 44/21., 45/21., // 21-Sc42,43,44,45
    44/22., 45/22., 46/22., 47/22., // 22-Ti44,45,46,47
    46/23., 47/23., 48/23., 49/23., // 23-V46,47,48,49
    48/24., 49/24., 50/24., 51/24., // 24-Cr48,49,50,51
    51/25., 52/25., 53/25., // 25-Mn51,52,53
    57/28., 58/28., 59/28., // 28-Ni57,58,59
    61/30., 62/30., 63/30., // 30-Zn61,62,63
    64/31., 65/31., // 31-Ga64,65
    68/33., 69/33., // 33-As68,69
    72/35., 73/35., // 35-Br72,73
    74/36., 75/36., 78/36., // 36-Kr74,75,78
    76/37. // 37-Rb76
  };

  TGraph *g = new TGraph(n, p, z);
  g->SetMarkerColor(2);
  g->SetMarkerStyle(20);
  TCanvas *c = new TCanvas("c", "calibration of Z", 800, 600);
  g->Draw();
  g->Fit("pol1");
} // end function fitZ
